Venetian blinds typically comprise a plurality of horizontal slats suspended beneath a headrail by two or more flexible ladders. The ladders each include a pair of vertically extending side tapes interconnected by a plurality of vertically spaced slat supporting rungs, and the upper ends of the ladders are attached to a ladder carrier or tilt drum to tilt the slats in response to turning of the ladder carrier. In order to equalize motion of the several ladders, the ladder carriers for the several ladders are rotated in unison by a tilt rod.
In order to move the venetian blind slats between an open position in which the slats are disposed in a generally horizontal plane and a closed position in which the slats are disposed at a shallow angle to a vertical plane, it is necessary that the tilt drum or ladder supports, be capable of moving the side strips of the ladders relative to each other through a distance approximating the width of the slats. Some tape drums, for example as disclosed in U.S. Pat. Nos. 4,494,593 and 5,341,865 are formed with an oblong cross-section and arrange the ladder side strips on the drum so that the major transverse dimension of the drum is generally horizontal when the blind is open and generally vertical when the blind is closed. With such tape drums, the headrail must have a height sufficiently greater than the major transverse dimension of the drum, to not only accommodate turning of the drum, but also provide space below the drum for lift cords used for raising and lowering the blind. The tape drums in some other blinds such as disclosed in U.S. Pat. No. 3,918,513, wrap the upper ends of the side strips of the ladder in relatively opposite directions and through one or more wraps around a small diameter drum so that the drum can be turned through more than 180 degrees to raise one of the ladder side strips while lowering the other of the ladder side strips between a blind open and a blind closed position. If the drum has a small diameter as compared to the width of the slats, then the upper portions of the ladder tapes converge at an acute angle above the upper slat. This tends to cause the upper slat to hang-up in a tilted condition when the tape drum is operated from a blind closing position to a blind opening. Further, the drum limits the minimum space between the ladder side strips when in the blind closing position and inhibits full closing of the slats.
Prior ladder supports generally raise one ladder side strip and lower the other ladder side strip by equal amounts, when moving the blind between an open condition and a closed condition. With such ladder supports, a light gap commonly occurs between the upper edge of the slat and the under side of the headrail, when the blind is in a closed condition.